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From Nick Dimiduk <ndimi...@gmail.com>
Subject Re: [Shadow Regions / Read Replicas ] Block Affinity
Date Tue, 03 Dec 2013 23:46:45 GMT
On Tue, Dec 3, 2013 at 11:37 AM, Enis Söztutar <enis.soz@gmail.com> wrote:

> I think we do not want to differentiate between RS's by splitting them between
> primaries and shadows. This will complicate provisioning, administration,
> monitoring and load balancing a lot, and will not achieve very cheap
> secondary region promotions (because you have to move the region still as
> you described).
>

The idea of having "primary hosts" and "replica hosts" was brought up in
initial design discussions over here. I am particularly against this
approach because of the additional complexity. I need to update myself on
Enis's doc (I'm a week+ behind), but my opinion is that we treat a
non-primary region (be it a "read replica" or a "shadow region") as a
first-class and independent entities. These entities can be assigned to any
host in the cluster, each with their own individual state machine instances.

Of course, the balancer would need to be aware of the relationship between
the primary and its non-primaries in order to maintain the balancing policy
requirements. However, I see no reason for there to be specialization at
the host level, and I agree with Enis's arguments against it.

-n

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